Abscisic acid (“ABA”) is a naturally occurring plant growth regulator that regulates a wide range of plant physiological processes such as seed germination, seedling elongation, abiotic stress response, flowering, and fruit development. The naturally occurring and biologically active form of ABA is the S enantiomer, (S)-abscisic acid (“(S)-ABA”). Consequently, a variety of commercial utilities have been identified for (S)-ABA in horticulture and agronomy. (S)-ABA exerts its biological activities by binding to (S)-ABA receptors and activating cellular signal transduction cascades. In addition, (S)-ABA has been demonstrated to have pharmaceutical and nutraceutical utilities (see U.S. Pat. No. 8,536,224).
Synthetic analogs of ABA may exhibit biological activities either similar to (S)-ABA but with altered (enhanced) potency (ABA agonists) or with a differing spectrum of affinity for the multiple ABA receptors than (S)-ABA itself has. The synthetic analogs may also possess improved uptake by plant tissues as well as enhanced metabolic stability. Additionally, synthetic analogs may have better chemical and environmental stability than (S)-ABA. Thus, synthetic ABA analogs may possess unique biological activities and have been pursued as an approach to identify novel plant growth regulators.
A variety of synthetic analogs of ABA have been revealed in the public domain. Several Japanese research groups have synthesized ABA analogs with modifications of the side chain and/or with cyclohexenone ring substituents through de novo synthesis (Y. Todoroki, et al. Phytochem. 1995, 38, 561-568; Y. Todoroki, et al. Phytochem. 1995, 40, 633-641; S, Nakano, et al. Biosci. Biotech. Biochem. 1995, 59, 1699-176; Y. Todoroki, et al. Biosci. Biotech. Biochem. 1994, 58, 707-715; Y. Todoroki, et al. Biosci. Biotech. Biochem. 1997, 61, 2043-2045; Y. Todoroki, et al. Tetrahedron, 1996, 52, 8081-8098). Synthesis of (S)-3′-halogen-ABA, (S)-3′-azido-ABA and (S)-3′-alkylthio-ABA from (S)-ABA have also been reported (Y. Todoroki, et al. Tetrahedron, 1995, 51, 6911-6926; S. Arai, et al. Phytochem. 1999, 52, 1185-1193; J. J. Balsevich, et al. Phytochem. 1977, 44, 215-220; Y. Todoroki, et al. Tetrahedron, 2000, 56, 1649-1653; Y. Todoroki, et al. Bioorg. Med. Chem. Lett. 2001, 11, 2381-2384). The work done by S. R. Abrams and coworkers at the Plant Biotechnology Institute at National Research Council of Canada is also noteworthy. Using de novo synthesis approaches, ABA analogs with modified side-chains or C6′-substitution have been prepared either as racemic mixtures or, in some cases, as pure stereoisomers (see U.S. Pat. No. 5,518,995; D. M. Priest, et al. FEBS Letters, 2005, 579, 4454-4458). A tetralone series of analogs in which the cyclohexenone ring of (S)-ABA is replaced with a bicyclic tetralone ring have also been described (J. M. Nyangulu, et al. Org. Biomol. Chem. 2006, 4, 1400-1412; J. M. Nyangulu, et al. J. Am. Chem. Soc. 2005, 127, 1662-1664; WO2005/108345).
The synthetic ABA analogs reported in the literature are limited in scope and are often prepared via multi-step de novo synthesis. The syntheses generally suffer from low overall yields, particularly when the optically pure single enantiomers are desired. Thus, these compounds are generally expensive to synthesize in large amounts or to manufacture on a commercial scale, limiting their commercial application. The (S)-ABA analogs of the present invention possess the aforementioned biological activities and, more importantly, can be prepared efficiently from (S)-ABA, which until recently was not available in large quantities.
The biological activity of racemic (±)-3′-methyl-ABA has been briefly described in a publication (K. Ueno, et al. Bioorg. Med. Chem. 2005, 13, 3359-3370), but the synthesis of this compound has not been reported. According to Ueno, et al., (±)-3′-methyl-ABA showed equal activity to (S)-ABA in a rice seedling elongation assay and weaker activity than (S)-ABA in an (S)-ABA 8′-hydroxylase inhibition assay. In addition, a structure that could be interpreted as representing (S)-3′-methyl-ABA was printed in a paper (Y. Todoroki, et al. Bioorg. Med. Chem. Lett, 2001, 11, 2381-2384), but neither the synthesis nor any biological data for that compound has been described in the public domain. Thus, there is no prior art in the public domain that enables the synthesis of (S)-3′-methyl-ABA and esters thereof or teaches the biological activities of (S)-3′-methyl-ABA and its esters. Most importantly, it was not obvious to those of skill in the art at the time of the invention, based on all available information in the public domain (vide supra), that (S)-3′-methyl-ABA would offer any advantage over (S)-ABA or (±)-3′-methyl-ABA.
Accordingly, there is a need for entantiomerically pure (S)-3′-methyl-ABA analogs which may be agonists of (S)-ABA and have improved biological activity. There is also a need for methods to prepare (S)-3′-methyl-ABA and esters thereof.